When somebody says "Good for you" it can genuinely mean you are happy for the person, or it can be used sarcastically where it would mean you don't care much about what they just said. For example: if you were helping a friend study for a test and they received a grade of 90%, you could say "Good for you!". By saying this, you are genuinely happy about their grade. If you want to use it sarcastically: if you received a low test score and someone comes to you and brags about receiving a grade of 90%, you can reply "Good for you..." in a sarcastic-voice (monotonous) to show you don't really care about their grade.

You will say this to a person to whom something nice has happened or she is in a better situation and you wanna tell her that you are happy for her. -Anita: Guess what??!!!!!!!! I have found a new job. :) -Ted: wow, what a surprise!!! Good for you. You may use "good for you" in a different way as well. -You need to know what is good for you with this illness; below are some instructions help you to recover faster.